Typical types of conventional frequency-current conversion circuits are shown in FIG. 1 and FIG. 2. In the frequency-current conversion circuit shown in FIG. 1, a capacitor 38 is charged to a predetermined value during the half cycle of a square-shaped input pulse signal having frequency proportional to the rate of rotation of the engine, and then capacitor 38 is discharged to drive an ammeter 27 during the next half cycle. The wave form of the current flowing through the ammeter 27 is shown in FIG. 5, in which the ordinate I.sub.M represents the current flowing through the ammeter, the abscissa t represents the time, and T represents one cycle interval of the input signal.
In the circuit shown in FIG. 2, a monostable multivibrator generates an output which is a series of voltage pulses, each of which is synchronized with a series of square-shaped pulses and has a uniform pulse width. An ammeter 27 is driven by the output of the monostable multivibrator. The wave form of the current flowing through the ammeter 27 is shown in FIG. 6. Since the output current signal of each circuits shown in FIG. 1 and FIG. 2 is a series of pulses, the pointer of the ammeter indicating the rate of rotation vibrates when the frequency of the output current signal is extremely low. Moreover in the circuit shown in FIG. 1, since the pulse width of the output current depends on a time constant determined by the capacitance of a capacitor 38 and the internal resistance of the ammeter 27, the charge of the capacitor 38 begins anew before the completion of the discharge of the capacitor 38 when the rate of rotation of the engine rises. Therefore the circuit cannot avoid the occurrence of an error in the indication of the ammeter 27. In the circuit shown in FIG. 2, an output is a series of voltage pulses. Therefor when the internal resistance of the ammeter 27 varies in accordance with the ambient temperature, the current flowing through the ammeter 27 also varies, causing an error in the indication of the ammeter 27. For this reason a temperature compensation circuit is a necessity for this circuit.